Board Thread:Pokémon Help/@comment-31398341-20171026040947/@comment-31796688-20171103144615

Gengar does beat it though:

252 Atk Adaptability Mega Lucario Bullet Punch vs. 0 HP / 0 Def Mega Gengar: 60-72 (44.4 - 53.3%) -- 23.4% chance to 2HKO

252 SpA Mega Gengar Shadow Ball vs. 0 HP / 4 SpD Mega Lucario: 109-130 (75.1 - 89.6%) -- guaranteed 2HKO